1. Field of the Invention
The present invention relates to an adjustment device for a vehicle seat such as an automotive seat and, more particularly, to such adjustment device which is capable of adjusting the height and depth of a seat cushion, the angles of inclination of a seat back, the longitudinal positions of a headrest and the like.
2. Description of the Prior Art
In a vehicle seat, there are provided various adjustment devices to adjust the height of a seat cushion, the angles of inclination of a seat back, the longitudinal positions of a headrest and the like, so that an occupant of the seat is able to take a desired sitting position according to his or her taste or physical condition.
Conventionally, as the adjustment device of this kind, there have been proposed and enforced various kinds of devices, in which link members supporting movable members to be adjusted are rotated using, for example, a shaft of rotation provided with a screw mechanism or a brake mechanism, or a gear mechanism. For example, in Japanese Patent Publication No. 77930 of 1984, there is disclosed an adjustment device in which a screw rod is rotated to rotate link members so that a seat cushion can be moved forwardly or rearwardly. Also, in Japanese Utility Model Publication No. 144738 of 1982, the inclination of a seat back of a vehicle seat can be adjusted by a shaft of rotation provided with a brake mechanism, and also in this Utility Model Publication there are employed a rack and pinion engaging with the rack.
However, the above-mentioned conventional adjustment devices of this kind have been found inadvantageous in some respects. That is, the adjustment devices employing the screw mechanism and the gear mechanism are complicated in structure and not easy to operate because the operation members thereof must be operated to a great extent. The devices employing the shaft of rotation provided with the brake mechanism are easy to produce a loosend condition, since the brake mechanism is provided only on one side of the shaft of rotation, resulting in insufficient locking.